At the hockey game, Eddie and Dillon arrive and sit together. Eddie, wearing his jacket, goes to get beer for them. When he arrives back at their seats, Eddie's coat is folded and hanging on the rail in front of their seats.
Eddie's wife carries in a box of Apple Jacks cereal from the dining room and places it on a high shelf above the kitchen sink, right next to another box of cereal, as Eddie comes home and washes his hands in the sink. But when Eddie is shown grabbing some food out of the fridge in an alternate angle shot of the same scene, the cereal boxes are down on the sink behind the dish rack. Still, the Apple Jacks logo is front and center.
When Eddie and Dillon are at the Bruins/Blackhawks hockey game, a Hawks player is pulled down by a Bruin near the Chicago goal. Visible in the background is the Chicago goalie wearing jersey number 35. (This was Tony Esposito in the 1970s.) There is a cut to Eddie and Dillon in the stands and then a long shot showing the subsequent on-ice altercation. The Chicago goalie is involved in this brawl, but he is now wearing jersey number 1 (which, in those days, belonged to Gary Smith, the Hawks backup goalie).
Towards the end of the final scene, Dillon and Foley are standing and talking to each other almost face-to-face. Then in the cut to the final shot they are suddenly standing at a very different angle to each other.
When Eddie and his wife are in the kitchen, she asks him if he wants breakfast, and he says, "no" but proceeds to put two slices of bread in the toaster. In the next scene, the bread is toasted, which takes only 18 seconds.
At the hockey game, when Dillon (Peter Boyle) and his young accomplice are discussing Eddie (Robert Mitchum) after Eddie goes for beers, two women, a few rows down and to the far left, keep looking over at the actors.
When Eddie meets Dave Foley in the park and says, "Hell, I'm 51 years old," his lips don't match that statement.
When leaving the Bruins game, they are driving through either the Sumner or Callahan tunnel. Their destination was the South Shore, so they would not be using either of those tunnels since they would point them toward the airport (or away from it which would be impossible considering their starting point).
When the robbers are at the beach parking lot heading out to the house of the bank manager for the last bank robbery before they are caught by Foley at the house, they exit the parking lot and take a right, which would bring them further into South Boston. The house they were caught at is in either Milton or Randolph so they should have turned left out of the lot which would have brought them past the state police barracks and onto the Southeast Expressway to head south to Milton or Randolph. The direction they went would bring them further away from their destination.
When Eddie talks with Dillon in the bar just after the bank robbers are caught, a mike is visible in the upper part of the screen.
When the young man who sells the guns to Eddie is sitting in his Road Runner with the man who will steal the machine guns from the Armory, the military man asks if the car has a "Hemi" engine. The young man says yes. Hemi refers only to the 426-cubic-inch engine. The Road Runner in the movie clearly has a "383" badge on the hood scoop. This engine is not a Hemi.
In the second bank heist, a security camera can be seen, but it doesn't have duct tape over it like the ones in the first heist.
There are minor errors in the local accent or lingo. Foley's boss (Sauter?) refers to the town of Natick (actually pronounced NAY-tick, with the long "a") as "Nattick" (with a short "a"). Also, near the end, Dillon says, "Take the Memorial Drive across the Mass. Avenue bridge." A local would simply say "Memorial Drive" (no "the") and "Mass Ave."